Networks conforming to the Institute of Electrical and Electronic Engineers (IEEE) 802.11 protocol (Nov. 18, 1997) use two protocols, the DCF (Distributed Coordination Function) and PCF (Point Coordination Function) for channel arbitration. The DCF performs well under low load situations. The PCF is optimal under high load conditions. The DCF works better in networks where Basic Service Sets (BSSs) overlap, the PCF is ideally suited for networks where BSSs are carefully planned not to overlap. The DCF has a relatively low implementation complexity, whereas the PCF is reputed to be more complex to implement. The DCF does not allow explicit access control, the PCF does. The DCF efficiency drops considerably in densely populated BSSs, the PCF has no scaling problem.
Due to the inability of the PCF to work well under overlapping Basic Server Set (BSS) conditions and the high implementation complexity, the PCF has not yet been widely adopted in current 802.11 implementations. The demand for better medium efficiency and a versatile QoS (Quality of Service) platform, however, increased interest in this optional access mechanism of the 802.11 MAC (Media Access Control).
The hybrid nature of the 802.11 MAC has caused proposals to focus either on the DCF or the PCF. However, by only looking at the PCF and not considering the DCF overlooks the fact that the 802.11 MAC always spends some time under the DCF access mechanism rules and that the DCF is also an integral part of a PCF based system. The system always has to spend at least a small part of its time under the DCF. The PCF has the fundamental characteristic that a station can't access the medium unless explicitly polled. However, to be polled, the station must first make itself known to the Point Coordinator, which requires medium access. Therefore, a PCF based solution should support both contention-free and contention periods. A contention period is useful for bursty traffic, adjacent BSSs, probe requests, association and re-association requests, etc. Latency is introduced if the channel is overloaded in the contention period.
Delay sensitive applications, such as VoIP (Voice over Internet Protocol), require short DTIM (Delivery Traffic Indication Message) intervals (e.g. 30 milliseconds) to minimize CF (Contention Free) polling latency. A fast DTIM beacon rate wastes bandwidth because of the beaconing overhead and because contention-based transmissions cannot span the TBTT (target beacon transmission time). A fast DTIM beacon rate also requires power-save Wireless Stations (WSTAs) to wake up more often, for example to receive multicast frames and buffered unicast frames.
In installations with multiple QoS applications with different service rates, the DTIM beacon rate cannot match the sampling rate for each application. Actually, it is difficult to match the sampling rate for any application. It is not efficient to arbitrarily poll WSTAs in every CFP (Contention Free Period). Periodic polling is not optimal for intermittent traffic. For example, VoIP traffic can be intermittent due to silence suppression.
Depending on the ‘load of the medium’, the system may spend more or less time in the CFP. In a heavily loaded system, the system may spend the larger part in the CFP while a mildly loaded system may spend the larger part in the CP (Contention Period). The balance between the two access mechanisms is a function of the medium load. As a consequence, both access mechanisms must provide the same QoS capabilities. The transition between one access mechanism and the other must be a smooth one. This is especially a challenge in average loaded systems where the DCF efficiency is starting to breakdown while the PCF efficiency is not yet optimal. For the upper layer protocol (or application) the performance profile of the service should be linear over all medium conditions and this is something that should be considered when proposing a PCF based system. Therefore, when proposing PCF enhancements, one also to consider the interaction between the PCF and the DCF and the dynamics of the system as a whole under various medium load conditions.
PCF combines the ability of full medium control with optimal medium efficiency, without suffering from scalability problems. However, there are two issues that limit the use of the current PCF for QoS systems. Section 9.3.4 and specifically clause 9.3.4.1 of the IEEE 802.11 standard imposes strict rules upon the order in which stations are addressed or polled. This is undesirable in a QoS system. Secondly, there is no mechanism, other than the More-Data bit, that allows a station to communicate its queue states to the PC.
Another concern is that PCF and DCF applications do not always coexist well. The PCF model only supports “polled” inbound transmissions during a CFP. As a result long PCF-based CFPs can starve DCF-based stations. The problem is exacerbated when CFPs in overlapping BSSes must be scheduled to avoid CFP contention. PCF polling is appropriate for isochronous applications, but DCF is more appropriate for asynchronous data. It should not be assumed that PCF polling is used for all high-priority inbound transmissions.